Within the past 15 years or so, modern optical techniques have been applied to the field of fluid mechanics, for example, for the purpose of analyzing the velocity of fluid flow and/or the distribution of particles or bubbles within the fluid.
The following three articles provide background of interest as regards the present invention. N. Ben-Yosef et al, in an article entitled Bubble size distribution measurement by Doppler velocimeter, Journal of Applied Physics, Vol. 46, No. 2, Feb. 1975, describe an experiment during which a laser Doppler velocimeter (LDV) was utilized to measure the velocity and size distribution of gas bubbles in water based upon the light scattering properties of the bubbles as they moved through the velocimeter laser beam from a bubble generator introduced into the bottom of a water column through appropriate mechanical fitting. Similarly, W. W. Martin et al, in their article Characteristics of Laser-Doppler Signals from Bubbles, appearing in the Int. J. Multiphase Flow, Vol. 7, pp. 439-460, (1981), also report on experiments aimed at measuring the signal characteristics of light scattered from single bubbles rising in quiescent water by means of laser-Doppler velocimetry. In this experiment, bubbles were investigated in the range of 200-1,000 micrometers and, as in the Ben-Yosef et al experiment, were generated by an air nozzle introduced at the bottom of the water column. This particular reference provides a detailed explanation of the triple-peaked laser-Doppler signal characteristic of the passage of a bubble through a dual-beam laser-Doppler velocimeter configuration. An article by W. Lauterborn et al entitled Modern Optical Techniques in Fluid Mechanics, Ann. Rev. Fluid Mech., pp. 223-244, published by Annual Reviews Inc. (1984) describes various optical techniques used in fluid mechanics to observe and measure properties of flow fields such as velocities and densities, and which describes briefly (at pp. 239 et seq.) the laser-induced formation of bubbles within a liquid.
Also of background interest is U.S. Pat. No. 4,348,111 to Goulas et al which discloses an optical particle analyzer that a generates a broad laser beam with a concentric narrow beam and a second, displaced narrow beam. The intensity and duration of the broad light beam reflected by individual particles within a particle stream indicates particle size. The concentric narrow beam is used to validate the passing of a particle through the broad beam's center. Time delay between interception of the concentric and displaced narrow beams is used to determine particle velocity. U.S. Pat. No., 4,211,487 to Morrison et al discloses an apparatus for determining aerosol size distribution, utilizing laser technology. A first laser directs an amplitude modulated beam through the aerosol distribution in question, such that droplets of a particular size are sequentially resonated. A second laser beam is directed against the oscillating droplets, is scattered thereby and then monitored in order to analyze droplet size distributions within the aerosol.